A power converter can be used to balance and control the phases of a three phase alternating current (AC) network.
A 2- or 3-level converter includes a common direct current (DC) capacitor used to exchange energy between all three phases. JP 2000083387 discloses a control method of a 2- or 3-level converter, in the form of a three phase inverter, in which three-phase AC is generated by alternately repeating a switching control (two-arm-on switching) in which an upper arm of a certain phase is turned on, the lower arm of the other phase is turned on, and both the upper arm and lower arm are turned off for a remaining phase among each phase of the three-phase voltage type inverter. This results in a uniform application of voltage and keeps voltage at the virtual neutral point of the delta loop at zero potential.
With a chain-link converter, each phase includes its own DC capacitors and energy transfer between phases must be done by means of an additional current or voltage. Chain-link converters do not have the flexibility to freely generate negative-sequence current as the classical 2- or 3-level converters have. The three-phase chain-link converter has three phase legs where each phase has a number of series-connected cell converters. The phase legs of series-connected cell converters can be connected either in delta or in wye (star) configuration.
For a wye-connected converter, it is possible to generate a pure positive-sequence current when the network system voltage contains a negative-sequence component, i.e., when the feeding network experiences unsymmetrical fault conditions. This is achieved by controlling the converter neutral point to a value corresponding to the negative-sequence voltage of the network.
For delta-connected converter, this is not possible since there is no real neutral point in a delta configuration. Instead, a circulating current inside the delta connection (i.e. a zero-sequence current) is used to balance the power between phase legs of the chain-link converter. A zero-sequence current is also used when the converter shall generate a negative-sequence current during balanced voltage conditions or when the system voltage is not balanced and a current shall be generated by the converter.
WO 2010/145706 discloses an arrangement for exchanging power with a three-phase electric power network comprises a voltage sourced converter (VSC) having three phase legs (A-C) with each a series connection of switching cells. The three phase legs are interconnected by forming a delta-connection. The arrangement also comprises a control unit configured to calculate a value for amplitude and phase position for a zero-sequence current for which, when circulated in the delta-connection circuit of said three phase legs, the balance of the total direct voltage of each of said three phase legs (A-C) with respect to the other two phase legs is restored will there be an unbalance and control the semiconductor devices of switching cells of the phase legs to add such a zero-sequence current to the currents of each phase leg of the converter.